1. Field of the Invention
The present invention generally relates to a switching apparatus and, more particularly, is directed to a signal transmission reception switching apparatus.
2. Description of the Prior Art
Generally, in a cordless telephone, a telephone communication is carried out in a so-called bidirectional fashion so that the transmission of its own audio signal and the reception of an audio signal from the other party are constantly performed simultaneously.
To this end, as shown in FIG. 1, in the cordless telephone, a transmitting circuit 1 and a receiving circuit 2 are connected to an antenna 4 through a separating circuit 3 which is what might be called a duplexer.
The duplexer 3 is generally composed of a bandpass filter whose pass band is the frequency of a transmitting signal and a bandpass filter whose pass band is the frequency of a receiving signal. This method utilizing the duplexer 3 is effective when the transmitting frequency and the receiving frequency are considerably distant from each other.
In a digital mobile telephone set, for example, a digital mobile telephone now commercially available on the market of the United States, the transmitting frequency and the receiving frequency are 800 to 900 MHz, respectively, and a difference between the transmitting frequency and the receiving frequency is 45 MHz at any one of the channels. Accordingly, if the transmitting circuit 1 and the receiving circuit 2 are connected through the duplexer 3 to the antenna 4, cut-off frequency characteristics of the bandpass filters of the duplexer 3 must be made steep because the transmitting frequency and the receiving frequency are both high and they are close to each other. However, the bandpass filters having such steep cut-off frequency characteristics cannot be produced without difficulty from a technological standpoint, which increases the loss of signals and the increases of size, weight and cost of the digital mobile telephone.
In the digital mobile telephone system, although the telephone conversation is performed in a bidirectional manner, the transmission of its own audio signal and the transmission of the audio signal from the other party are performed in a time division multiple access (TDMA) system. Therefore, as shown in FIG. 2, the transmitting circuit 1 and the receiving circuit 2 are switched by a change-over circuit (switching circuit) 5 and then can be connected to the antenna 4.
The switching circuit 5 is generally constructed as, for example, shown in FIG. 3.
In FIG. 3, reference numerals 11, 12 and 13 denote PIN (positive-intrinsic-negative) diodes, and 21, 22, 23 and 24 1/4 wavelength coaxial lines, respectively. Upon transmission, the diode 11 is turned OFF and the diodes 12, 13 are turned ON, whereas upon reception, the diode 11 is turned ON and the diodes 12, 13 are turned OFF.
In the switching circuit 5 shown in FIG. 3, however, the diodes 11, 12 and 13 have a relatively large resistance value so that Q is lowered, thus making it impossible to obtain a high isolation.
Further, since the transmission signal is passed through the diode 12, which has a large resistance value, a transmission loss occurs in the transmission signal, thereby causing an antenna output to be lowered. Furthermore, since the isolation is low, the transmission signal is also leaked to the receiving circuit 2 side, thereby increasing the transmission loss.
Alternatively, in order to obtain a proper antenna output, the transmitting circuit 1 must transmit a larger transmission output, which is disadvantageous for a power supply source battery. Furthermore, since the transmission loss in the diode 12 is large, the diode 12 must be able to withstand a large voltage, which increases a manufacturing cost of the switching circuit 5.
In addition, since a loss occurs in the receiving signal due to the diode 13, a reception sensitivity is lowered.